fix: remove ActiveSupport dependency from fail_if_new default

ENV["CI"].present? crashes when ActiveSupport core extensions aren't
loaded yet. Discovered when integrating with jetthoughts.github.io
which loads the gem before Rails. Use plain Ruby nil/empty check.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: pftg

lib/capybara_screenshot_diff.rb

@@ -63,7 +63,7 @@ def screenshot_area_abs
     module Diff
       mattr_accessor(:delayed) { true }
       mattr_accessor :area_size_limit
-      mattr_accessor(:fail_if_new) { ENV["CI"].present? }
+      mattr_accessor(:fail_if_new) { !ENV["CI"].nil? && !ENV["CI"].empty? }
       mattr_accessor(:fail_on_difference) { true }
       mattr_accessor :color_distance_limit
       mattr_accessor(:enabled) { true }